The present invention generally relates to data communications, and more particularly, to an apparatus and method for increasing the dynamic range of a receiver by implementing a double echo canceller at the input.
In the field of data communications, a transceiver, or modem, is used to convey information from one location to another. Digital subscriber line (DSL) technology now enables DSL transceivers to more rapidly communicate data than previously possible with purely analog modems. DSL transceivers communicate by modulating a baseband signal carrying encoded digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a multi-dimensional multi-level signal space constellation and slicing the received constellation to recover the transmitted information. The constellation can include both analog and digital information or only digital information. FIG. 1 is a block diagram of a prior art communications system, wherein the transmitted signals in the two opposite directions are mixed and separated at the hybrid connectors.
Since DSL transceivers use the public switched telephone network (PSTN) and other similar networks, DSL systems are subject to echo cancellation problems with respect to voice band users of the PSTN. Echo is harmful to successful DSL signal delivery as it significantly degrades signal quality. Therefore, an echo cancellation technique should be employed to separate the upstream and downstream signals in DSL transceivers.
The dynamic range of the electrical components at the input of the receiver in a communications system may limit a receiver""s performance. The dynamic range is determined by the maximum peak to peak allowed at the input and by the number of bits of, for example, an A/D converter at the input of the receiver. The performance is also limited by the quantization noise of the system, which is determined by the number of bits used in the various processes. If, for example, the resolution of the processor that follows the A/D is lower than the resolution of the A/D, the total resolution of the receiver is determined according to the processor""s resolution. This may result in the loss of a substantial portion of a transmitted data signal. Because the range of the A/D can limit the performance of the receiver as a whole, there is a need to improve the dynamic range of the A/D thereby improving the dynamic range of the receiver itself.
The noise that may interfere with the communications system of FIG. 1 may result from echo in the system. Line echoes (i.e., electrical echoes) occur in telecommunications networks due to impedance mismatches at hybrid transformers that couple two-wire local customer loops to four-wire long-distance trunks. Ideally, the hybrid passes the far-end signal at the four-wire receive port through to the two-wire transmit port without allowing leakage into the four-wire transmit port. However, this would require exact knowledge of the impedance seen at the two-wire ports, which in practice varies widely from individual circuit to individual circuit and can only be estimated. Consequently, the leaking signal returns to the far-end transmitter as an echo. The situation can be further complicated by the presence of two-wire toll switches, allowing intermediate four-two-four wire conversions internal to the network. In telephone connections using satellite links, with round-trip delays on the order of 600 ms, line echoes can become particularly disruptive.
Additionally, echo and other noise may be generated as the result of bridge taps in the communications system. A bridge tap is an undetermined length of wire attached between the normal endpoints of a circuit that introduces unwanted impedance imbalances into a communications system. A bridge tap is not on the direct electrical path between the central office and a user""s location. In situations involving bridge taps, the noise at the receiver may effectively cancel the transmitted signal.
Echo suppressors have been developed to control line echoes in telecommunications networks. Echo suppressors de-couple the four-wire transmit port when signal detectors determine that there is a far-end signal at the four-wire receive port without any near-end signal at the two-wire receive port. Echo suppressors, however, are generally ineffective during double-talk when speakers or modems at both ends of the system are talking or transmitting simultaneously. During double-talk, the four-wire transmit port carries both the near-end signal and the far-end echo signal. Furthermore, echo suppressors tend to produce speech clipping, especially during long delays caused by satellite links.
Echo cancellers generally include an adaptive filter and a subtracter. The adaptive filter attempts to model the echo path. The incoming signal is applied to the adaptive filter, which generates a replica signal. The replica signal and the echo signal are then applied to the subtracter. The subtracter removes the replica signal from the echo signal to produce an error signal. The error signal is fed back to the adaptive filter, which adjusts its filter coefficients (or taps) in order to minimize the error signal. In this manner, the filter coefficients converge toward values that optimize the replica signal in order to cancel (i.e., at least partially offset) the echo signal. Echo cancellers offer the advantage of not disrupting the signal path. Economic considerations place limits on the fineness of sampling times and quantization levels in digital adaptive filters, but technological improvements are relaxing these limits.
Echo occurs primarily because of the impedance mismatch between the hybrid connector and the two-wire phone line. An echo canceller operates by first estimating the parameters of the echo path, and then combining the estimate with the transmitted data, thus emulating the echo. This emulated echo (also referred to herein as echo replica) is then subtracted from the received signal, which ideally results in an echo-free transmission.
Accordingly, there is a need for a way to recover and preserve the transmitted signal that may be accompanied by a substantial amount of noise and echo. There is also a need to improve the dynamic resolution of the receiver to better protect the received signal that may otherwise be lost in the unwanted echo and noise. Accordingly, a solution that overcomes the shortcomings of the prior art is desired.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with one aspect of the invention, an apparatus is provided having a transmit path and a receive path for communicating data. The apparatus includes an analog to digital (A/D) converter that is disposed in communication with the receive path. The A/D converter operates at a first sampling rate, and it converts a received analog signal into a digital signal. The apparatus also includes a first echo canceller that is in communication with the receive path and that operates at the first sampling rate for estimating a first portion of an echo signal leaking from the transmit path to the receive path. The estimated echo signal is subtracted from the digital signal. The amplitude of the digital signal is increased by a digital gain. A decimator is disposed in communication with the receive path, whereby the decimator filters the digital signal that has a first sampling rate and emits a signal output at a reduced sampling rate. Thereafter, a second echo canceller is in communication with the output from the decimator for generating an echo replica estimation of a second portion of the echo signal leaking from the transmit path to said receive path. The second portion of the echo signal leaking from the transmit path to the receive path is subtracted from the output from the decimator. The result is a substantially echo-free digital data signal.
In accordance with another aspect of the invention, a method for increasing the dynamic range in a receiver in a communications system is provided. The method includes the steps of receiving a analog signal on a receive path for communicating data. The received analog signal is converted to a digital signal at a first sampling rate. An estimation of a first portion of an echo signal leaking from a transmit path in the receiver to the receive path is made and subtracted from the converted digital signal. The sampling rate of the converted digital signal is thereafter reduced to a predetermined sampling rate. An estimation of a second portion of an echo signal leaking from the transmit path in the receiver to the receive path is made. This second portion of the echo signal is subtracted from the converted digital signal at the reduced sampling rate.